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Background: Know: SNMP over UDP, Trap, Response, SNMP versions, hierarchy of managers , Recognize: sysUpTime , coldStart =Reliability of notifications= Problem: Trap message may be lost Trap message may be lost and agent doesn't know whether it needs to resend the trap resulting to manager's unawareness of event 1. : Reasons e.q : a.the underlying transport might not be reliable and the trap information might thereby be dropped on the way to the manager. b.the trap might fail to reach its destination when the network is congested. Trap is a way for an agent to tell the NMS (Management entities) that something abnormal has happened. And it is used to convey an event by the agent to a manager2. In this way, The trap originates from the agent and is sent to NMS and no acknowledgement is made from NMS to agent. Therefore the agent has no way of knowing if the trap arrives NMS and doesn't know it needs to resend the trap. This situation makes the structure of agent simple but undesirable in a hierarchy of managers where traps cannot be forwarded up because lower manager has resources to retransmit. Solution: SNMPv2 introduces InformRequest SNMPv2 provides an''' inform mechanism''', which allows for manager-to-agent communication. This operation can be useful when the need arises for more than one NMS in the network( What's in a name: "Simple"). When a trap is sent from an agent to NMS, or an inform is sent from one NMS to another, the receiver sends a response to the sender acknowledging receipt of the event. This behavior is similar to that of the get and setrequests(see more in GetRequest, SetRequest). In this way agents will know whether they need to resend the trap when no response from manager is received. The implement of confirmed notifications needs more complexity than with the uncomfirmed ones (SNMPv1)because the new agent needs to buffer and retian a memory of notifications that were emitted and manage what to do in case that an acknowledgement is not received. In addtion, the inform request is not primarily intended for use between device-based agents and management applications, but rather for communications between management applications when one application temporarily plays the role of an agent3. Here are some pictures illustrating the comprision between trap and inform request: When a SNMP agent sends a trap message to a SNMP manager. The agent doesn't know whether the trap reaches the manager side even though the manager receives it. When the trap is lost due to some reasons, no response will be aquired so that the manager can't know what happened from the agent. And the agent won't resend the trap since it knows nothing about the condition at manager. When first time the SNMP manager receives the inform request from the agent, it soon sends a "response" message back to the agent. So the inform request(trap) will not be sent again. When the agent sent an inform request to the manager but didn't receive any response back from the manager, it will resend the request to make sure the manager will know this notification. After the manager receives the inform request and it sends a response to agent so no addtional inform request is sent. Configure the Router to Send Traps4 To understand better, there are some examples: 1. Send the SNMP and Cisco environmental monitor enterprise-specific traps to address 172.30.2.160: snmp-server enable traps snmp-server host 172.30.2.160 public snmp envmon 2. Enable the router to send all traps to the host myhost.cisco.com using the community string public: snmp-server enable traps snmp-server host myhost.cisco.com public 3. Enable the router to send all inform requests to the host myhost.cisco.com using the community string public: snmp-server enable traps snmp-server host myhost.cisco.com informs version 2c public Reference 1A manager might later detect a missing coldStart/warmStart trap (Generic trips) when it notices that the sysUptime is not what it expects, given knowledge of previous startup times. 2Network Management Fundamentals,Alexander Clemm PhD.Chatper 5:Management Functions and Reference Model:Getting Organized.P133-137 3Network Management Fundamentals,Alexander Clemm PhD.Chatper 8:Common Management Protocols:Languages of Management:.P259 4Cisco IOS Softwree Release 11.3 SNMP Inform Requests Category:All